Traditional grammar defines the object in a sentence as the entity that is acted upon by the subject.[1] There is thus a primary distinction between subjects and objects that is understood in terms of the action expressed by the verb, e.g. Tom studies grammar - Tom is the subject and grammar is the object. Traditional theories of sentence structure divide the simple sentence into a subject and a predicate,[2] whereby the object is taken to be part of the predicate.[3] Many modern theories of grammar (e.g. dependency grammars), in contrast, take the object to be a verb argument like the subject, the difference between them being mainly just their prominence; the subject is ranked higher than the object and is thus more prominent.[4]

The main verb in a clause determines whether and what objects are present. Transitive verbs require the presence of an object, whereas intransitive verbs block the appearance of an object.[5] The term complement overlaps in meaning with object: all objects are complements, but not vice versa. The objects that verbs do and do not take is explored in detail in valency theory.

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Various object types are commonly acknowledged: direct, indirect, and prepositional. These object types are illustrated in the following table:

Type

Description

Example

Direct object

Entity acted upon

Sam fed the dogs.

Indirect object

Entity indirectly affected by the action

She sent him a present.

Prepositional object

Object introduced by a preposition

She is waiting for Tommy.

The descriptions "entity acted upon" and "entity indirectly affected by the action" are merely loose orientation points. Beyond basic examples such as those provided in the table, these orientation points are not much help when the goal is to determine whether a given object should be viewed as direct or indirect.[6] One rule of thumb for English, however, is that an indirect object is not present unless a direct object is also present. A prepositional object is one that is introduced by a preposition. Despite the difficulties with the traditional nomenclature, the terms direct object and indirect object are widespread.

The term oblique object is also employed at times, although what exactly is meant varies from author to author. Some understand it to be an umbrella term denoting all objects (direct, indirect, and prepositional), whereas others use the term to denote just a prepositional object.[7]

Languages vary significantly with respect to these criteria. The first criterion identifies objects reliably most of the time in English, e.g.

Fred gave me a book.

a. A book was given (to) me. - Passive sentence identifies a book as an object in the starting sentence.

b. I was given a book. - Passive sentence identifies me as an object in the starting sentence.

The second criterion is also a reliable criterion for isolating languages such as English, since the relatively strict word order of English usually positions the object after the verb(s) in declarative sentences. The third criterion is less applicable to English, though, since English lacks morphological case, exceptions being the personal pronouns (I/me, he/him, she/her, they/them). For languages that have case and thus freer word order, morphological case is the most readily available criterion for identifying objects. In Latin and related languages, direct objects are usually marked with the accusative case, and indirect objects with the dative case. Note as well that some objects are marked in telling ways in particular languages. In Spanish, for example, human objects have to be marked by the preposition a; the phenomenon is called differential object marking.

Ergative[11] and object-deletion verbs[12] can be transitive or intransitive, as indicated in the following table:

Transitive

Example

Ergative

The submarine sank the freighter.

Object deletion

We have already eaten dinner.

Intransitive

Example

Ergative

The freighter sank.

Object deletion

We have already eaten.

The distinction drawn here between ergative and object-deletion verbs is based on the role of the subject. The object of a transitive ergative verb is the subject of the corresponding intransitive ergative verb. With object-deletion verbs, in contrast, the subject is consistent regardless of whether an object is or is not present.

Objects are distinguished from subjects in the syntactic trees that represent sentence structure. The subject appears (as high or) higher in the syntactic structure than the object. The following trees of a dependency grammar illustrate the hierarchical positions of subjects and objects:[13]

The subject is in blue, and the object in orange. The subject is consistently a dependent of the finite verb, whereas the object is a dependent of the lowest non-finite verb if such a verb is present.